Reading the ECG
IT'S FAR MORE IMPORTANT TO LOOK AT THE PATIENT THAN TO LOOK AT THE ECG
You can have patients who have fairly normal looking ECG tracings, but are actually needing immediate intervention. On the contrary you can have patients whose ECG's look erratic and worrying, but they are fine in person. This could be due to e.g. a lead falling off of the chest or the patient coughing or sneezing while the tracing was being conducted.
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In order to read the ECG correctly the speed of the tracing must be set to 25mm/s and the gain set to 10mm/mv
Timings on the ECG
The X-axis of the ECG corresponds to time and the Y axis to voltage.
- Each large square on the graph corresponds to 0.2 seconds. Hence 5 large squares is 1 second
- Each small square on the graph corresponds to 0.04 seconds hence 5 small squares make a large square
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- Check the timing of each of the components of the ECG wave to make sure they are the correct length
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PR interval
0.12 to 0.2s (one large square)
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QRS Complex
Less than 0.12s (three small squares)
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QT interval
0.4s (two large squares) to 0.46s (max)
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Here you are able to see the size of the large squares on the trace. Each of these squares corresponds to 0.2 seconds. The large squares are made of 5 smaller squares in length. Each of the small squares represents 0.04s in real time
This image shows a zoomed in view with the normal length of each of the wave components showing. The squares in this image are the small squares each representing 0.04s. Five of these small squares would form one large square (shown on the zoomed out image) Image credit: http://www.research.chop.edu/programs/lqts/ecg.php
Here you are able to see the size of the large squares on the trace. Each of these squares corresponds to 0.2 seconds. The large squares are made of 5 smaller squares in length. Each of the small squares represents 0.04s in real time
Stepwise Interpretation
The anagram ARI BAR is a useful way of remembering all the different steps. But again the first and most important step is to:
Observe the Patient
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Then preceed to check the ARI BAR questions:
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-Any electrical activity?
-Rate?
-Irregular or Regular rhythm?
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-Broad QRS complexes?
-Are there any P-Waves?
-Relationship between P-Wave and QRS complex?
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Once you have done this check the length of the PR segment, QRS and QTc, you also should comment on axis (details in intermediate tutorial) and note any other features that can be seen e.g. ST elevation, T wave inversion etc
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Any Electrical Activity?
Check the ECG to see if there is electrical activity on every one of the leads.
If any areas are flatlined then this can suggest that one of the leads has fallen off or there has been an arrest.
This is a normal ECG trace in sinus rhythm with electrical activity in all leads. Compare it to the next image
In this ECG trace notice that there is a flatline in the limb lead II and II areas. This means that one of the stickers holding the leads on has fallen off. Can you work out which one (Left leg - green lead)
This is a normal ECG trace in sinus rhythm with electrical activity in all leads. Compare it to the next image
Rate?
There are two different ways to establish the heart rate:
1) Rate = 300 / (no. of big squares between QRS peaks)
2) Rate = number of QRS peaks in 30 large squares x 10
Here we divide 300 by 3 (number of big squares between the QRS peaks). This gives a rate of 100bpm NOTE: you can only use this method if the rhythm is regular and it only gives an estimated bpm
Here we count off 30 large squares and count how many QRS peaks fit in this. Then we multiply that number by 10. In this case there are 9 peaks in 30 squares and so the rate is 9 x 10 = 90 bpm. NOTE: you need to use the rhythm strip to count off the squares NOTE: it doesn't matter if the rhythm is irregular you can still use this method NOTE: see how we get a slightly different and more accurate rate using this method than the last
Here we divide 300 by 3 (number of big squares between the QRS peaks). This gives a rate of 100bpm NOTE: you can only use this method if the rhythm is regular and it only gives an estimated bpm
Irregular or Regular Rhythm?
The rhythm can be described in 3 ways
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Regular (metronome like with every beat equally spaced)
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Regularly Irregular (every beat not equaly spaced but there is a distinct pattern)
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Irregularly Irregular (sporadic pattern of beats with no clear spacing)
This is a regular rhythm. Look at the rhythm strip here and use a separate piece of paper to mark off the top of the QRS complexes. If you measure the gap between them you should see that they are all practically the same. In this case they are all just over 3 large squares in size
This is an example of ventricular ectopics which are extra beats added in to a normal rhythm. The third beat here (marked with a thicker line) is an ectopic beat and hence this rhythm is called trigeminy. Notice how the the two normal beats are all equally spaced apart but the spacing between the ectopic beat and the second normal beat is smaller. This pattern is irregular, but because it repeats itself it is known as a regularly irregular pattern
Here there is no reproducible pattern of spacing between the beats. This pattern is irregularly irregular and is the ECG tracing of atrial fibrillation
This is a regular rhythm. Look at the rhythm strip here and use a separate piece of paper to mark off the top of the QRS complexes. If you measure the gap between them you should see that they are all practically the same. In this case they are all just over 3 large squares in size
Broad QRS Complexes?
Normally it takes less than 0.12 seconds for ventricular depolarisation to happen and so the QRS complex should be narrow (smaller than 3 small squares).
If the QRS is broader than this, that suggests there is something causing a slowing of conduction through the ventricles e.g. a bundle branch block or ventricular tachycardia.
Are There Any
P-Waves
Check all the different leads to establish whether there is a presence or absence of P-waves.
Absence of P-waves may suggest some of the following conditions
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Atrial Fibrillation
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Sinus Arrest
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Hyperkalaemia
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Sinoatrial Block
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Ventricuar Tachycardia
See the absence of P-waves
Note the absence of P-waves
See the absence of P-waves
Relationship Between P-wave and QRS
It's important to check that there is a P-wave for every QRS complex and that there is a QRS complex for every P-wave.
By doing this you can see if there are any extra beats or dropped beats
This is an example of 3rd degree heart block or complete heart block as it is known. The signal from the SAN can't reach the ventricles and the ventricles only contract by the spontaneous depolarisation of the AV node. This means that the p-waves and QRS complexes don't match up because the p-wave isn't initiating the QRS like normal
This is an example of what's known as Wenkebach phenomenon where the PR interval gradually increases with every beat until eventually there is a P-wave which isn't conducted through to the ventricles and the QRS complex never occurs. (The orange circles in the diagram show where the dropped QRS complex occurs)
Here there is a normal 1:1 relationship between the P-waves and QRS
This is an example of 3rd degree heart block or complete heart block as it is known. The signal from the SAN can't reach the ventricles and the ventricles only contract by the spontaneous depolarisation of the AV node. This means that the p-waves and QRS complexes don't match up because the p-wave isn't initiating the QRS like normal